Systems for manufacturing commercial products of rare earth magnet include a single part system wherein a part of substantially the same shape as the product is produced at the stage of press molding, and a multiple part system wherein once a large block is molded, it is divided into a plurality of parts by machining. These systems are schematically illustrated in FIGS. 1A and 1B. FIG. 1A illustrates the single part system including press molding, sintering or heat treating, and finishing steps. A molded part 101, a sintered or heat treated part 102, and a finished part (or product) 103 are substantially identical in shape and size. Insofar as normal sintering is performed, a sintered part of near net shape is obtained, and the load of the finishing step is relatively low. However, when it is desired to manufacture parts of small size or parts having a reduced thickness in magnetization direction, the sequence of press molding and sintering is difficult to form sintered parts of normal shape, leading to a lowering of manufacturing yield, and at worst, such parts cannot be formed.
In contrast, the multiple part system illustrated in FIG. 1B eliminates the above-mentioned problems and allows press molding and sintering or heat treating steps to be performed with high productivity and versatility. It now becomes the mainstream of rare earth magnet manufacture. In the multiple part system, a molded block 101 and a sintered or heat treated block 102 are substantially identical in shape and size, but the subsequent finishing step requires cutting. It is the key for manufacture of finished parts 103 how to cutoff machine the block in the most efficient and least wasteful manner.
Tools for cutting rare earth magnet blocks include two types, a diamond grinding wheel inner-diameter (ID) blade having diamond grits bonded to an inner periphery of a thin doughnut-shaped disk, and a diamond grinding wheel outer-diameter (OD) blade having diamond grits bonded to an outer periphery of a thin disk as a core. Nowadays the cutoff machining technology using OD blades becomes the mainstream, especially from the aspect of productivity. The machining technology using ID blades is low in productivity because of a single blade cutting mode. In the case of OD blade, multiple cutting is possible. FIG. 2 illustrates an exemplary multiple blade assembly 1 including a plurality of cutoff abrasive blades 11 coaxially mounted on a rotating shaft 12 alternately with spacers (not shown), each blade 11 including a core 11b in the form of a thin doughnut disk and an abrasive grain layer 11a on an outer peripheral rim of the core 11b. This multiple blade assembly 1 is capable of multiple cutoff machining, that is, to machine a block into a multiplicity of parts at a time.
For the manufacture of OD abrasive blades, diamond grains are generally bonded by three typical binding systems including resin bonding with resin binders, metal bonding with metal binders, and electroplating. These cutoff abrasive blades are often used in cutting off of rare earth magnet blocks.
When cutoff abrasive blades are used to machine a rare earth magnet block of certain size into a multiplicity of parts, the relationship of the cutting part (axial) width of the cutoff blade is crucially correlated to the material yield of the workpiece (magnet block). It is important to maximize a material yield and productivity by using a cutting part with a minimal thickness, machining at a high accuracy to minimize a machining allowance and reduce chips, and increasing the number of parts available.
In order to form a cutting part with a minimal width (or thinner cutting part) from the standpoint of material yield, the cutoff wheel core must be thin. In the case of OD blade 11 shown in FIG. 2, its core 11b is usually made of steel materials from the standpoints of material cost and mechanical strength. Of these steel materials, alloy tool steels classified as SK, SKS, SKD, SKT, and SKH according to the JIS standards are often used in commercial practice. However, in an attempt to cutoff machine a hard material such as rare earth magnet by a thin OD blade, the prior art core of alloy tool steel is short in mechanical strength and becomes deformed or bowed during cutoff machining, losing dimensional accuracy.
One solution to this problem is a cutoff wheel for use with rare earth magnet alloys including a core of cemented carbide to which high hardness abrasive grains such as diamond and cBN are bonded with a binding system such as resin bonding, metal bonding or electroplating, as described in JP-A 10-175172. Use of cemented carbide as the core material mitigates buckling deformation by stresses during machining, ensuring that rare earth magnet is cutoff machined at a high accuracy. However, if a short supply of cutting fluid is provided to the cutting part during machining of rare earth magnet, the cutoff wheel may give rise to problems like dulling and loading even when a core of cemented carbide is used, which problems increase the machining force during the process and induce chipping and bowing, providing a detrimental impact on the machined state.
Approaches to address this problem include arrangement of plural nozzles near the cutoff blades for forcedly feeding cutting fluid to the cutting parts and provision of a high capacity pump to feed a large volume of cutting fluid. The former approach is quite difficult to implement in combination with a multiple blade assembly including a plurality of blades arranged at a close spacing of about 1 mm because nozzles cannot be arranged near the blades. In the latter approach of feeding a large volume of cutting fluid, the air streams created around the cutting parts during rotation of the cutoff blades cause the cutting fluid to be divided and scattered away before it reaches the cutting parts. If a high pressure is applied to the cutting fluid to forcedly feed it, the pressure is detrimental to high-accuracy machining because it causes the cutoff blades to be bowed and generates vibration.
To solve these problems, improved methods for cutoff machining a rare earth magnet block have been proposed which methods can feed a small amount of cutting fluid to points of cutoff machining in an efficient manner and achieve cutoff machining at a high speed and a high accuracy as compared with the prior art.
One process of multiple cutoff machining a rare earth magnet block involves providing a multiple blade assembly including a plurality of cutoff abrasive blades mounted on a rotating shaft at axially spaced apart positions, and rotating the plurality of cutoff abrasive blades. A cutting fluid is effectively fed to the plurality of cutoff abrasive blades by providing a cutting fluid feed nozzle having a plurality of slits corresponding to the plurality of cutoff abrasive blades such that an outer peripheral portion of each cutoff abrasive blade may be inserted in the corresponding slit. Then the slits serve to restrict any axial run-out of the cutoff abrasive blades during rotation. At the same time, the cutting fluid reaching the slit and coming in contact with the outer peripheral portion of each cutoff abrasive blade is entrained on surfaces of the cutoff abrasive blade being rotated and transported toward the peripheral cutting part of the cutoff abrasive blade by the centrifugal force of rotation. As a result, the cutting fluid is effectively delivered to points of cutoff machining on the magnet block during multiple cutoff machining.
When cutoff grooves corresponding to the plurality of cutoff abrasive blades are formed in the surface of the magnet block, each cutoff groove serves to restrict any axial run-out during rotation of the cutoff abrasive blade whose outer peripheral portion is inserted in the cutoff groove. The cutting fluid flowing from each slit in the feed nozzle and across the surfaces of the cutoff abrasive blade flows into the cutoff groove and is then entrained on the surfaces of the cutoff abrasive blade being rotated whereby the cutting fluid is effectively fed to the blade cutting part during multiple cutoff machining.
Also a jig including a pair of jig segments for clamping the magnet block in the machining direction for securing the magnet block is proposed wherein the jig segments are provided on their surfaces with a plurality of guide grooves corresponding to the cutoff abrasive blades so that the outer peripheral portion of each cutoff abrasive blade may be inserted into the corresponding guide groove. Then the guide grooves serve to restrict any axial run-out of the cutoff abrasive blades during rotation. The cutting fluid flowing from each slit in the feed nozzle and across the surfaces of the cutoff abrasive blade flows in the guide groove and is then entrained on the surfaces of the cutoff abrasive blade being rotated whereby the cutting fluid is effectively fed to the blade cutting part during multiple cutoff machining.
In either case, cutoff machining of the magnet block can be performed at a high accuracy and a high speed while effectively feeding a smaller volume of cutting fluid than in the prior art to points of cutoff machining.
Nevertheless, the current desire for more efficient manufacture of rare earth sintered magnet entails a propensity to enlarge the size of magnet blocks to be cutoff machined, indicating an increased depth of cut. When a magnet block has an increased height, the effective diameter of the cutoff abrasive blade, that is, the distance from the rotating shaft or spacer to the outer periphery of the blade (corresponding to the maximum height of the cutoff abrasive blade available for cutting) must be increased. Such larger diameter cutoff abrasive blades are more liable to deformation, especially axial runout. As a result, a rare earth magnet block is cut into pieces of degraded shape and dimensional accuracy. The prior art uses thicker cutoff abrasive blades to avoid the deformation. Thicker cutoff abrasive blades, however, are inconvenient in that more material is removed by cutting. Then the number of magnet pieces cut out of a magnet block of the same size is reduced as compared with thin cutoff abrasive blades. Under the economy where the price of rare earth metals increases, a reduction in the number of magnet pieces is reflected by the manufacture cost of rare earth magnet products.